To investigate the hyperfine interaction in the ES, a fine frequency scanning of the ES peaks was performed. Fig.REF  reveals that the resonance in ES is actually composed of a few peaks. In comparison with the hyperfine splitting of the GS spin levels by three equivalent nitrogen nuclear spins [1]}, the ES spin levels also show a same pattern of seven peaks but with larger interaction \(\approx \)  90 MHz ((Fig.REF a). This reflects the larger density of the ES wavefunction near the N nucleus. In addition to coupling to the N nucleus, one can aslo observe a few more dips inside each peak with a frequency distance around 18.8 MHz ((Fig.REF b). This is probably due to the hyperfine interaction to the Boron nucleus that is a bit further away than the Nitrogen.
